1. Field of the Invention
The present invention relates to a method for preventing defects such as holes from being formed on surfaces of blank tubes manufactured by mandrel mill, wherein the holes are produced during the manufacturing of the seamless steel tubes, and more particularly during the manufacturing seamless steel tubes made of a high alloy steel such as stainless steel and has a shell wall thickness-to-outside diameter ratio of not greater than 0.025.
2. Description of the Related Art
In one method of manufacturing seamless steel tubes using a mandrel mill, a heated billet is pierced by a piercing machine, and a finishing rolling process of the billet is applied by rolling the inside of the tube. As is shown in FIG. 4, the mandrel mill employed in this circumstance normally comprises a plurality of--from five to eight--roll stands 1 configuring a roll groove having a plurality of rolls 2 and 2' in alternate pairs arranged horizontally and vertically.
These plurality of grooved roll stands are disposed orthogonally about a rolling shaft, and a mandrel bar 3 is disposed within a roll groove formed by roll stands 1. The inner surface of the blank tube 4 is rolled by the mandrel bar 3.
In the manufacturing of seamless steel tubes by mandrel mill, holes are often formed on surfaces of the blank tubes 4 during the rolling process causing unfavorable defects in the mandrel mill manufacturing. (Hereinafter, this defect is referred to as the "hole defect" and reference numeral 6 indicates the portions with the "hole defect.")
Conventionally, it was thought that the "hole defect" was caused by the following reasons. As is shown in FIG. 5, when a blank tube made of a high alloy steel, such as stainless steel, is rolled by the mandrel mill at a temperature ranging between 950.degree. C. and 1050.degree. C. which is the normal rolling temperature range for common blank tubes, the hot-working characteristics of the blank tube deteriorates.
When a blank tube having an inferior hot-working characteristic is rolled by the mandrel mill, a longitudinal tensile force is exerted only on a flange portion 5 shown in FIG. 2 of the blank tube receiving no reduction, which eventually causes a rupture or "hole defect" in the tube. These defects tend to occur with much greater frequency in steel tubes having a thin wall thickness. This tendency is particularly remarkable in products which have shell wall thickness-to-outside diameter ratio of not greater than 0.0.25.
Various method for preventing the "hole defect" have been proposed.
One of the common methods for preventing the "hole defect" is disclosed in Japanese Patent Laid-Open Publication No.58-224155, wherein a method of improving the hot-working deformability of the rolling tube materials is proposed.
There is also proposed, in Japanese Laid-Open Publication No.63-84720, a method of reducing the rolling reduction of one stand where the "hole defect" occurs in the mandrel mill and dispersing the reduction load to the remaining stands, and of reducing the wall thickness of blank tubes at the entrance of the mandrel mill so as to reduce the rolling reduction of each stand of the mill.
The method disclosed in Japanese Patent Laid-Open Publication No.58-22455, however, cannot provide a sufficient hot-working deformability at rolling temperatures in the range of 950.degree. C. to 1050.degree. C. in the mandrel mill.
Although the method proposed in Japanese Patent Laid-Open Publication No.63-84720 can prevent the "hole defect", it may not be used on blank tubes with a thin wall thickness for the following reason:
If after reduction loads are dispersed to each stand, and there remains a stand in which the rolling load exceeds the reference value, the wall thickness of the blank tube is reduced at the entrance of the mandrel mill so as to reduce the rolling reduction of each stand of the mill. However, rolling reduction on the blank tube by the piercing machine is limited and therefore the wall thickness of the blank tube cannot be reduced below a lower limit. In the above situation, it is difficult to roll blank tubes with a thin wall thickness.
Accordingly, an object of the present invention is to overcome the above described problems of the mandrel mill and prevent the "hole defect."